Shear Key Design

[xduncanx]

Hi,

We use a shear key in the output shaft of a motor to prevent catastrophic failure of a mechanism. However, sometimes the key shears but the mechanism still able to drive to catastrophic failure despite the key clearly in two pieces.

The current assembly method to retain the key is to drive a grub screw down through the driven gear boss onto the key surface. I believe this is tightened considerably and could be the reason why we still have substantial drive after key failure.
What are some guidelines when designing shear keys and how to retain them?

thanks,
Michael

 

[MikeHalloran]

Assuming you're talking about a simple square/rectangular key in classical keyseats, the grub screw seems a terrible idea.

... at least partly because it removes any radial space into which either piece of the key can retreat in order to stop torque transmission.

... i.e., the grubbed key works as intended only if the fracture surface of the sheared key is perfectly cylindrical at the shaft surface radius. Any deviation from such a surface provides sort of a secondary key which can drive the load. A FBD of the unfractured key should suggest that its fracture surface is more likely planar; in that circumstance, the key/shaft interface takes on a 'D' shape that can still transmit torque.

It probably looked good on paper to whichever drafter originated the concept. You might try a more strict engineering analysis to help you, and us, understand the nature of the fractures you can find. ... and you should attempt to recover some of the fractured keys and mating parts, and post photographs for all to see.


Retention is probably why, e.g., Briggs & Stratton uses a Woodruff key between shaft and flywheel. It's retained by the shaft's arcuate keyseat until it fractures. It's also made of soft-ish aluminum so it can be quickly rubbed smooth by relative motion after key fracture.

Other intentional shear key mechanisms typically provide fairly brittle keys, stressed in single or double shear, and some clearance between the mating parts near the intended shear surface, so the shear surface doesn't have to be perfect, and some radial space in which the key fragments can move away from each other so that an irregular fracture surface cannot transmit torque.

For examples, disassemble a commercial shear-key hub for power transmission, or the propeller drive on a really old outboard motor.




Mike Halloran
Pembroke Pines, FL, USA